PSI - Issue 8

M. Barsanti et al. / Procedia Structural Integrity 8 (2018) 501–508

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M. Barsanti et al. / Structural Integrity Procedia 00 (2017) 000–000

Fig. 2. Schematic design of the Maraging samples for mechanical tests.

is subjected to deformation at a constant and low speed, with values between 10 − 7 and 10 − 4 s − 1 . The local strain rate should be low enough to prevent certain corrosion or di ff usion processes to occur [ASTM (2013)]. Samples from the groups 2-4 were mechanically tested for a better understanding of the hydrogen content e ff ect on the deterioration of mechanical properties. Samples were prepared with the schematic design shown in Fig. 2, in conformity with the ASTM E8 / E8M-16a, standard [ASTM (2016)] used for traction test sample preparation. Standard characterization of tensile properties of the samples was performed using MTS hydraulic testing machine (MTS Systems Corporation, Eden Prairie, MN USA) equipped with 100 kN load cell and MTS 634.31 F25 axial extensometer (base length 10 mm). The data acquisistion system was RT3 (Trio Sistemi e Misure S.r.l., Dalmine (BG), Italy). Metallographic examination was carried out on material samples surfaces (normal to the rolling direction). Surfaces were grinded using SiC papers up to 800 grit and then mirror polished. The observation was performed using a standard optical microscope (LEICA DMI 3000 M, Leica Microsystems, Wetzlar (Germany)). The fracture surface of the charged samples was observed using a Scanning Electron Microscope to have an indi cation of the hydrogen penetration profile and brittle area. The surface condition of the blades was inspected using SEM / EDS (Energy Dispersive Spectroscopy) technique (JEOL JSM 5600LV). Fractographic investigations have been performed on the cross-section of the fractured blade using OM–Leica DM300 to define the crack pattern. Furthermore, the fracture surfaces both of the failed blades and of the fractured mechanical specimens were characterized using the same technique. 2.4. Surface analysis

3. Results

3.1. Hydrogen charging and content measurements

The charging conditions are summarized in the first column of table 3; in the same table, the hydrogen contents of the samples are shown. Samples from broken blades were analyzed with the same technique, and showed hydrogen concentrations ranging from 1.2 to 5.3 ppm, depending on the sampling point.